1. Field of the Invention
The present invention relates to a soft handoff system, a mobile terminal equipped with the soft handoff system, and a soft handoff method, which can reduce call drop rate by allowing a handoff drop timer of a base station in an active set having poor signal quality to continue to run rather than to be reset when the mobile terminal receives a Handoff Direction Message (HDM) from a current base station in a traffic state.
2. Description of the Related Art
Handoff is the process in which a mobile terminal is handed from one cell to the next in order to maintain a radio connection with a corresponding communication network.
FIG. 1 is a schematic diagram showing a handoff process. As a mobile terminal moves from a base station “A” to a base station “B”, the signal strength of the base station “B” increases whereas that of the base station “A” decreases. In order to continue an ongoing call of the mobile terminal without interruption, it is necessary to transfer a traffic channel established with the mobile terminal from the base station “A” to the base station “B”. When the mobile terminal is located midway between the two base stations “A” and “B”, the signal strengths of the two base stations “A” and “B” are nearly equal. Accordingly, the traffic channel can be established between the mobile terminal and any one of the two base stations “A” and “B”. At this time, handoff is performed.
The mobile terminal determines when to handoff. Since the mobile terminal constantly monitors whether it approaches to neighbor base stations, it constantly measures pilot channel signal strengths Ec/Io of neighbor base stations. This procedure is called “pilot searching”. The mobile terminal constantly performs pilot searching while it is powered on, regardless of whether or not it is in communication.
A mobile terminal in communication analyzes periodically monitored wireless conditions (i.e., periodically measured signal strengths) of neighbor base stations, and transmits a Pilot Strength Measurement Message (PSMM) to a current base station if necessary.
Specifically, if the mobile terminal in communication determines that the pilot channel signal strength Ec/Io of a neighbor base station “B” is high enough (i.e., greater than a first threshold value T_ADD), the mobile terminal sends a PSMM to the current base station “A”. The PSMM is a message containing information about the pilot channel signal strength Ec/Io of the new base station “B”.
The current base station “A” checks the condition of the new base station “B” based on the PSMM received from the mobile terminal. If the current base station “A” determines that there is no problem in handoff to the new base station “B”, the current base station “A” allocates a new traffic channel to the new base station “B”, and then transmits a Handoff Direction Message (HDM), which is a message containing a command to perform handoff, to the mobile terminal.
The mobile terminal performs handoff operation upon receipt of the HDM from the current base station “A”.
Specifically, when receiving the HDM, the mobile terminal allocates a traffic channel to each of the current and new base stations “A” and “B”. In this case, the mobile terminal transmits a Handoff Completion Message (HCM), which indicates that the handoff operation has been successfully performed, to the current base station “A”.
In the meantime, the mobile terminal constantly monitors pilot channel signal strengths Ec/Io of the current and new base stations “A” and “B”. If it is determined that the pilot channel signal strength of the current base station “A” is low enough (i.e., less than a second threshold value T_DROP), the mobile terminal checks whether or not the pilot channel signal strength Ec/Io of the current base station “A” remains below the second threshold value T_DROP for a threshold interval T_TDROP. If the pilot channel signal strength Ec/Io of the current base station “A” remains below the second threshold value T_DROP for the threshold interval T_TDROP, the mobile terminal transmits a PSMM, which is a message containing information about the pilot channel signal strength Ec/Io of the new base station “B”, to the current base station.
Upon receipt of the PSMM, the current base station “A” determines that the mobile terminal no longer requires a traffic channel established between the mobile terminal and the current base station, and then transmits an HDM to the mobile terminal after removing the traffic channel.
Upon receipt of the HDM, the mobile terminal removes the traffic channel established between the current base station “A” and the mobile terminal and transmits an HCM to the new base station “B”, thereby completing the entire handoff process.
Meanwhile, in the conventional soft handoff method, when a mobile terminal receives an HDM from a base station, it resets a handoff drop timer in active set in which pilot channel signal strength Ec/Io is less than a predetermined value (for example, the second threshold value T_DROP).
Specifically, if a mobile terminal receives an HDM from a base station in active set in which pilot channel signal strength Ec/Io is less than the second threshold value T_DROP, the mobile terminal resets a handoff drop timer which is now in progress. Consequently, the mobile terminal tracks again the signal strength of the base station having poor signal quality for a threshold interval T_TDROP. Thus, it is not possible to immediately terminate a call connection to a base station having poor signal quality.